1. Field of the Invention
The present invention relates generally to a constant current source and is directed more particularly to a transistor constant current source.
2. Description of the Prior Art
In a prior art constant current source shown in FIGS. 1 and 2, the following equation (1) is established between a base-emitter voltage V.sub.BE of a transistor used therein and its emitter current I.sub.E. EQU V.sub.BE =(kT/q)ln(I.sub.E /I.sub.s) (1)
where
k is the Boltzmann's constant; PA1 T is the absolute temperature; PA1 q is the charge of an electron; and PA1 I.sub.s is the saturated current in the reverse direction. PA1 I.sub.E1 is the emitter current of the transistor Q.sub.1 ; PA1 I.sub.E2 is the emitter current of the transistor Q.sub.2 ; PA1 A.sub.1 is the emitter-base junction area of the transistor Q.sub.1 ; and PA1 A.sub.2 is the emitter-base junction area of the transistor Q.sub.2. PA1 I.sub.2 is the collector current of the transistor Q.sub.2. PA1 R.sub.1 is the resistance value of a resistor R.sub.1 connected to the collector of the transistor Q.sub.1, the current I.sub.2 can be expressed from the equations (5) and (6) as follows: ##EQU3## PA1 V.sub.BE1 is the base-emitter voltage of the transistor Q.sub.1 ; PA1 V.sub.BE2 is the base-emitter voltage of the transistor Q.sub.2 ; and PA1 R.sub.3 is the resistance value of a resistor R.sub.3 connected to the emitter of the transistor Q.sub.2. PA1 (A) first, second, third and fourth transistors of one conductivity type each having base, emitter and collector electrodes; PA1 (B) a voltage supply source having first and second voltage terminals; PA1 (C) circuit means for connecting the collector and emitter electrodes of said first transistor to said first and second voltage terminals respectively with a first impedance means between the collector electrode and said first voltage terminal; PA1 (D) circuit means for connecting the emitter electrode of said second transistor to said second voltage terminal through a second impedance; PA1 (E) circuit means for connecting the emitter electrode of said third transistor to said second voltage terminal through a third impedance; PA1 (F) circuit means for connecting the emitter electrode of said fourth transistor to said second voltage terminal; PA1 (G) circuit means for connecting the base electrode of said first transistor to said emitter electrode of said second transistor; PA1 (H) circuit means for connecting said collector electrode of said first transistor to the base electrodes of said second and third transistors respectively; PA1 (I) circuit means for connecting said emitter electrode of said third transistor to the base electrode of said fourth transistor; and PA1 (J) current utilizing means connected between said first voltage terminal and at least one of the collector electrodes of said second, third and fourth transistors.
Between the saturated current I.sub.s in the reverse direction and an emitter-base junction area A of the transistor, established is the following equation (2). EQU I.sub.s =.gamma..multidot.A (2)
where .gamma. is a proportional constant.
In the prior art circuit of FIG. 1, since the base-emitter voltage of a transistor Q.sub.1 is equal to that of another transistor Q.sub.2, the following equation (3) is established from the equations (1) and (2). EQU (I.sub.E1 /I.sub.E2)=(A.sub.1 /A.sub.2) (3)
where
If the current amplification factor h.sub.FE of each of the transistors Q.sub.1 and Q.sub.2 is assumed sufficiently large, the base current thereof can be neglected. Therefore, the following relation (4) can be derived. ##EQU1## where I.sub.1 is the collector current of the transistor Q.sub.1 ; and
From the equations (3) and (4), obtained is the following equation (5) EQU (I.sub.2 /I.sub.1)=(A.sub.2 /A.sub.1) (5)
Since the following equation (6) is established on the transistor Q.sub.1, ##EQU2## where V.sub.CC is the voltage of a power source; and
Therefore, the transistor Q.sub.2 serves as a constant current source of the absorption type with the current represented by the equation (7).
With the above prior art circuit, since relation or ratio between the currents I.sub.1 and I.sub.2 is represented by the equation (5), if the ratio I.sub.2 /I.sub.1 is large, for example, the current I.sub.2 is selected large as 100 times as the current I.sub.1, it is necessary that the junction area A.sub.2 is selected 100 times of the junction area A.sub.1. Thus, the above prior circuit requires a large area and hence it is not suitable to be made as an IC (integrated circuit). While, in the case that the ratio I.sub.2 /I.sub.1 is small, if the current I.sub.2 is selected 1/100 of the current I.sub.1, the junction area A.sub.1 must be selected as large as 100 times of that A.sub.2. Thus, this case is not suitable as an IC, too.
In the prior art circuit of FIG. 2, the following equation (8) is established on the base of the transistor Q.sub.2. EQU I.sub.1 R.sub.1 +V.sub.BE1 =I.sub.2 R.sub.3 +V.sub.BE2 ( 8)
where
Since the following equation (9) is established, the equation (10) can be obtained from the equations (8) and (9). ##EQU4## where R.sub.2 is the resistance value of a resistor R.sub.2 connected to the emitter of the transistor Q.sub.1.
If the voltage drop across the resistor R.sub.1 is about the base-emitter voltage V.sub.BE, the second term in the brace of the equation (10) is small and hence neglected. Thus, the equation (10) can be considered as follows: EQU (I.sub.2 /I.sub.1).congruent.(R.sub.2 /R.sub.3) (11)
Accordingly, the current I.sub.2 can be expressed as follows: ##EQU5##
Therefore, the transistor Q.sub.2 functions as a constant current source of the absorption type with the current expressed by the equation (12).
Since, however, a resistor of an IC is generally formed by the diffusion of impurity, the area of the resistor in the IC is in proportion to the resistance value thereof. In the case of the constant current circuit of FIG. 2, since the relation between the currents I.sub.1 and I.sub.2 is represented by the equation (11), if the current I.sub.2 is selected, for example, 100 times of the current I.sub.1, the resistor R.sub.2 must be made to have the resistance value as 100 times as that of the resistor R.sub.3. That is, the area of the resistor R.sub.3 must be formed as 100 times as that of the resistor R.sub.2. Thus, the IC becomes large in area and hence the circuit of FIG. 2 is unsuitable as an IC, too.
FIG. 3 shows a practical circuit which is formed by using the constant current circuit of FIG. 2 to derive six constant current outputs I.sub.2 to I.sub.7. If the circuit of FIG. 3 is formed as an IC, the area occupied by one transistor in the IC is approximately equal to the area of a resistor with the resistance value of 2 K.OMEGA. which is formed by the diffusion of impurity. Therefore, the constant current circuit of FIG. 3 satisfies following values. EQU 112+1+1+1+4.8+17+33+100+2.times.6=281.8 EQU 281.8/2=140.9
That is, the circuit of FIG. 3 requires the area corresponding to a resistor of 281.8 K.OMEGA. or the area corresponding to 140.9 transistors.